This invention relates to integrated circuit fabrication and, more particularly, to a method of monitoring PGO spin-coating precursor solution synthesis using UV spectroscopy.
Lead Germanium Oxide (PGO or Pb5Ge3O11) thin films have broad application in ferroelectric 1T transistor devices. The methods of preparing PGO thin films include metal organic chemical vapor deposition (MOCVD), sputtering and spin-coating. For both MOCVD and sputtering methods, the thin film deposition processes are complex, and the equipment required is prohibitively expensive. Comparatively, the spin-coating technology is relatively simple and inexpensive.
The precursors for PGO ferroelectric thin film deposition are known in the art. The lead and germanium sources typically are lead acetate trihydrate (Pb(OAc)2.3H2O) and germanium alkoxide (Ge(OR)4 (wherein R=C2H5 or CH(CH3)2)). The organic solvents are 2-methoxyethanol and di(ethylene glycol)ethyl ether (DEGEE). In the prior art synthesis route, the first step is normally the distillation of Pb(OAc)2.3H2O in an organic solvent and then removal of the water. After mixing with the Ge(OR)4 organic solution, the PGO solution is further heated in a di(ethylene glycol)ethyl ether solution. The PGO solution is not heated in the 2-methoxyethanol composition due to solid precipitation during the distillation. During heating in the di(ethylene glycol)ethyl ether, the PGO solution may gradually change to a black color. The heating temperature may be as high as 180xc2x0 C. The PGO solution is believed to change to a black color when heated in Argon becuase black particles are believed to precipitate from the solution due to the reduction of Pb(2+).
According to Applicant""s experiments, there are several disadvantages to the reported PGO spin-coating precursor solution preparation. First, the 2-methoxyethanol PGO solution is not stable in air and is moisture sensitive. Second, in the DEGEE solution, the presence of a black color signifies decomposition of the PGO solution, which involves the reduction of Pb2+ to Pb+ (Pb2O). Third, monitoring the quality of the precursor is necessary. Therefore, problems remaining in the reported synthesis route include determining the correct PGO solution heat treatment method, determining the correct PGO precursor solution monitoring method, and determining the correct PGO solution spin-on property control method.
The present invention is directed toward providing a heat treatment method, controlling the properties of the synthesized solution, and monitoring the solution via UV spectroscopy so as to provide a suitable PGO spin-coating precursor solution. The method includes utilizing the starting materials of lead acetate trihydrate (Pb(OAc)2.3H2O) and germanium alkoxide (Ge(OR)4 (wherein R=C2H5 or CH(CH3)2)). The organic solvent is di(ethylene glycol)ethyl ether (DEGEE). The mixed solution of Pb(OAc)2.3H2O and DEGEE (the lead DEGEE solution) is heated in an atmosphere of air, not Argon, at a temperature no greater than 190xc2x0 C., and preferably no greater than 185xc2x0 C. for a time period in a range of thirty minutes to four hours, and preferably no greater than eighty-five minutes. The concentration of the solution typically is approximately 0.5 Normal. During the heating step the color of the solution is monitored by UV spectroscopy to determine when the reaction is complete and when decomposition of the desired reaction product begins to take place. A certain amount of DEGEE is then added into the lead DEGEE solution to adjust the solution to a desired lead concentration. To this lead DEGEE solution, a pre-mixed germanium DEGEE solution having a desired concentration is added to form a PGO precursor solution. The formed PGO precursor solution is then subjected to a second heating step of the process. This second step also entails heating the solution to a temperature no greater than 190xc2x0 C. for a time period in a range of 0.5 to 2.0 hours in an air atmosphere. The process results in a PGO precursor solution, having a desired concentration, suitable for use in spin-coating processes.
Accordingly, an object of the invention is to provide a method of synthesizing PGO ferroelectric spin-coating solutions.
Another object of the invention is to provide a method of determining the correct PGO solution heat treatment method.
Still another object of the invention is to provide a method of monitoring PGO ferroelectric precursor spin-coating solutions using UV spectroscopy.
A further object of the invention is to provide a method of controlling the properties of a PGO spin-coating solution using UV spectroscopy.